Nondestructive measurement of a film thickness is an important objective for many industries. For example, in the aircraft industry, paint for functional and decorative purposes is often applied to aircraft surfaces, which may have near critical weight. Also, prior to applying a surface paint, care must be taken to ensure that an adequate amount of primer is uniformly applied with an adequate thickness to act as a proper corrosion inhibitor. For example, in some aircraft applications, a paint film thickness of 0.4 to 1.0 thousandth of an inch (mil) over the surface of the substrate is desirable. Moreover, depending on the model of aircraft, over 500 pounds of primer and topcoat may be applied to an aircraft in accordance with customer specifications.
Since the weight and distribution of paint on aircraft surfaces may sometimes affect the performance of the aircraft it is desirable to apply and measure the paint in a controlled means in order to generally apply a minimum adequate thickness. This measurement is desirable to ensure that the primer is sufficiently thick for corrosion protection, but not so thick as to unnecessarily add to the weight of the airplane, which can negatively affect fuel efficiency. After the primer thickness is determined to be of sufficient thickness, a top coat of paint can be applied.
Aircraft components that are made of a metallic material can have their coatings measured by several conventional commercial tools. However, components are increasingly manufactured from carbon or glass fiber reinforced composite materials. These materials are difficult to measure using conventional techniques since they have relatively low direct-current conductivity and magnetic permeability.
Another conventional alternative is to wait for the paint to dry and indirectly measure the paint film thickness by weighing the component in question. This can result in an undesirable cycle delay of, for example, up to three hours. For example, when primer needs to be measured on a composite component, the component may first be weighed, then painted and finally reweighed. However, this process can be prohibitively difficult in the case of large components, for example, an airplane fuselage. Since an acceptably accurate and dependable direct measurement technology for composite materials is not available, when the primer film is suspected of being excessively thick or uneven, the component typically must be disassembled and repainted at significant cost and risk of damage.
Commercial ultrasonic measurement systems, such as Panametric 45L Deluxe®, are available which give some paint measurement thickness data for some types of composites. The Panametric 45L Deluxe® is useable for relatively thick coatings, but for thin coatings of paint this ultrasonic system is not sufficiently reliable and accurate to meet some specification requirements.
Additionally, the nondestructive measurement of film thickness is of concern to other industries, such as the automotive industry and the paper production industry. In the automotive industry, the analysis of primer film thickness is important, for example, to determine whether additional touchup work is required for some of the paint applications common to the industry. In the paper industry, verifying the uniformity of the paper thickness can be an important quality control issue. With the increasing use of composite materials, it has become apparent that current methods of measurement that perform adequately with metallic substrates or with thick films, but not with substrate materials that have relatively low direct-current conductivity and magnetic permeability or with thin films, are not sufficient.
Therefore, there is a need in various industries for systems and methods that are capable of nondestructive measurement of a thin film thickness over substrate materials that have relatively low direct-current conductivity and magnetic permeability.